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The Johnson 115 outboard motor is known for its quiet engine, good gas mileage and the fact that it seldom needs maintenance. One part you may need to replace, however, is the thermostat. The Johnson 115 utilizes the water it's submerged in as coolant. Your outboard's thermostat regulates its heating and cooling system by measuring the temperature of the motor, then activating heating and cooling systems as needed. If your motor begins to run hot on a regular basis it may mean the thermostat needs to be replaced.

Under The Hood:

How to Replace the Thermostat on a 4.3 V6 Mercruiser
How to Change the Thermostat in a 350 Engine
How to Change the Thermostat on a Johnson 115
How to Change a Thermostat on an LT1 Engine
How to Change a Thermostat in a Truck

How to Replace the Thermostat on a 4.3 V6 Mercruiser

Open the hood of your Mercruiser.

Locate the thermostat housing. The housing is on the top of the engine and has four hoses connected to it.

Remove the two bolts that secure the thermostat housing to the engine by using a ratchet and socket.

Remove the thermostat housing cover.

Remove the thermostat from the housing by pulling it out with pliers.

Clean the gasket mating surfaces with steel wool. Do not let debris get into the engine compartment.

Place the replacement thermostat into the housing. Install the new thermostat gasket that came with the thermostat kit by placing the gasket between the thermostat housing and the cover.

Secure the thermostat housing to the engine by installing and tightening the two bolts with a ratchet and socket.

Items you will need

Ratchet
Socket set
Pliers
Steel wool

How to Change the Thermostat in a 350 Engine

Locate the clamp on the end of the hose that connects the thermostat housing and the radiator. Loosen it and push it up towards the radiator. This may require the use of pliers or vice grips.

Disconnect the hose from the thermostat housing and move it out of the way. Leave the other end of the hose connected to the radiator. It is normal to lose a little bit of coolant during this process.

Remove the thermostat housing by locating the two bolts holding it in place. Use an open-end wrench to turn bolts in a counter-clockwise direction. Apply even, gentle pressure until it is removed. Do this carefully to avoid damaging the gasket that seals the housing to the 350 engine.

Remove the thermostat, which is clearly visible after removing the housing, and replace it with a new one specified for the 350 engine. Check with your automotive supplier to ensure you have the proper thermostat.

Replace the thermostat housing and tighten the two bolts in a clockwise direction. Be careful not to over or under tighten; either can cause coolant leakage and extreme heat stress to the engine.

Reconnect the hose from the radiator to the thermostat housing. Twisting one direction, then the other, in a small radius, will help ensure a good and tight fit. Bring the clamp back down the hose and tighten.

Remove the radiator cap and start the engine. This will release the air bubbles in the engine and hose. Once there are no more bubbles visible, replace the radiator cap and fill the coolant reservoir.

Items you will need

Open-end wrenches Vice grips Pliers Replacement thermostat Coolant Protective eyewear

How to Change the Thermostat on a Johnson 115

Remove the fasteners that hold your upper engine cover in place with a screwdriver. Pull the upper engine case off and set it and the fastener screws aside.

Locate the thermostat housing on the upper left hand side of the cylinder head. Remove the thermostat housing bolts with the socket set. Set the bolts and the housing aside.

Unscrew and remove the old thermostat. Use a wire brush to clean the contact points thoroughly.

Screw in the in the new thermostat, but not so tight that you damage the new thermostat. Replace the thermostat housing and bolt it back into place.

Replace the upper motor cover and the screws that hold it in place.

Items you will need

Screwdriver
Socket set
Wire brush
New thermostat

How to Change a Thermostat on an LT1 Engine

Remove the upper radiator hose from the thermostat housing by removing the retaining band that holds the hose to the thermostat housing. Pull the hose off the inlet spout. It's common to replace the older, spring-style band with a toothed band clamp. If this has been done, you will need a nut driver to remove the band clamp.

Remove the two bolts that hold the thermostat housing onto the LT1's intake manifold and lift up on the thermostat housing to remove it from the intake.

Pull the old gasket off the intake manifold and discard it. Pull the old thermostat out of the intake manifold of the LT1 and discard it. Neither item will be reused.

Wipe the mating surfaces of the intake manifold and the thermostat housing with a clean cotton towel to remove any old fluid and gasket material.

Cover the mating surface of the thermostat housing cover with a thick bead of RTV silicone gasket maker and let it set for 10 minutes, or follow the RTV manufacturer instructions located on the packaging.

Place a new thermostat into the intake manifold with the correct end facing upward. The top will be marked as such.

Place the thermostat housing onto the intake manifold and bolt it into place using the bolts you removed in Step 2. The RTV will squeeze out slightly. This is okay. Do not wipe it off.

Reattach the radiator hose with the original spring clamp or change it to a more modern band clamp.

Items you will need

LT1 thermostat
Wrench
RTV gasket maker
Screwdriver
Cotton towel

How to Change a Thermostat in a Truck

To Remove

Park your truck in a safe place and open the hood. Make sure the engine is cool enough to the touch. If it is at operating temperature, wait a few minutes for the engine to cool down.

Locate the thermostat housing on your engine. The best way is to follow the radiator upper hose towards the engine, then place a drain pan under the truck in the direction of the thermostat housing.

Remove the air duct or cleaner assembly to gain access to the thermostat housing if necessary. On some models, it is necessary to remove a steering pump-mounting bracket off the thermostat housing as well. Use a wrench or ratchet and socket to remove the bolts and bracket.

Unplug the sensor electrical connector from the thermostat housing if your model is equipped with one. Then remove the upper radiator hose from the thermostat housing by removing the clamp; use a Phillips screwdriver or ratchet and socket to loosen the clamp. Carefully twist and pull the hose off the housing and let the coolant drain onto the drain pan.

Remove the two thermostat housing mounting bolts using a wrench or ratchet and socket, then lift the housing off the engine and remove the thermostat and paper gasket or O-ring seal. Clean the housing and engine mounting surfaces of paper gasket material, using a plastic scraper if necessary.

To Install

Set the new thermostat in place with the spring pointing towards the engine. Install a new paper gasket or O-ring; apply a light coat of sealer to the gasket as recommended by the gasket manufacturer. Set the housing in place and start the mounting bolts by hand and then tighten the bolts.

Install the radiator upper hose and clamp over the thermostat housing. Also, install the power steering bracket and air cleaner assembly and plug the sensor electrical connector if you removed or disconnected them.

Start the engine and add 50 percent water and 50 percent antifreeze to the coolant reservoir. Let the engine reach operating temperature. Check for leaks and add coolant to the reservoir to bring the mixture level between the Add and Full Hot marks. Turn off the engine.

Items you will need

Drain pan
Wrench set
Ratchet and socket set
Phillips screwdriver
Plastic scraper
New thermostat gasket or O-ring
Gasket sealer